The increasing demand for higher performance power supply circuits has resulted in the continued development of voltage regulator and other power management devices. For example, many low voltage applications require the use of low dropout (LDO) regulators, such as for use in cellular phones, pagers, laptops, camera recorders and other wireless and mobile battery operated devices. These portable electronics applications typically require power management devices having a low voltage and quiescent current flow to facilitate increased battery efficiency and longevity. Such low dropout regulators generally provide a well-specified and stable dc voltage whose input to output voltage difference is low.
With reference to FIG. 1, an exemplary single channel amplifier circuit 100, such as an LDO, receiving an input voltage VIN and generating an output voltage VOUT through a load device RLOAD is illustrated. An enable pin ENABLE is used to power-up circuit 100 to turn on output voltage VOUT. Typically, enable pin ENABLE can be used to power-up circuit 100 in two manners.
In the first manner, enable pin ENABLE is connected to input voltage VIN to power-up circuit 100 and turn on output voltage VOUT. With reference to FIG. 2B, when power is turned on, input voltage VIN takes some amount of time to reach its steady level, possibly with some overshoot condition begin realized during power-up. Ideally, output voltage VOUT will be immune to what happens with input voltage VIN, ramping up in a smooth manner. However, in many instances output voltage VOUT will also realize overshoot conditions, which if greater than approximately 10% can be destructive. In the second manner, input voltage VIN is already turned on and established, and then ENABLE pin is enabled to turn on amplifier circuit 100. For example, with reference to FIG. 2, ENABLE pin and input voltage VIN have the same response curves upon connection, but output voltage VOUT can still tend to ramp up and exhibit destructive overshoot characteristics. Employing a bypass capacitor or other noise reduction capacitor can minimize the overshoot; however another concern is the ramp-up time that should be minimized for fast start-up LDO devices, with 50 microseconds or less in ramp-up time desirable.
For dual channel devices, the power-up process becomes more difficult to control. For example, with reference to FIG. 3A, a dual channel amplifier circuit 300 having a first error amplifier circuit 302 and a second error amplifier 304 is illustrated with dual enable pins ENABLE1 and ENABLE2 and a bandgap voltage VBG. Such dual channels are totally independent, with first enable pin ENABLE1 enabled and then second enable pin ENABLE2 enabled, or both pins ENABLE1 and ENABLE2 enabled together. Bandgap voltage VBG is powered up with the rest of amplifier circuit 300, e.g., when first enable pin ENABLE1 is enabled and error amplifier circuit 302 and output voltage VOUT2 are turned on. With reference to FIG. 3B, once enable pin ENABLE2 is enabled (referencing a fixed reference voltage VIN), output voltage VOUT2 ramps up without being suitably controlled and thus realizes overshoot conditions, exceeding not only bandgap voltage VBG but also input voltage VIN before settling. For example, with reference to FIG. 4, an amplifier circuit 400 such as used within a dual channel circuit can comprise an error amplifier 402 configured with a bandgap voltage VBG and feedback voltage VFB in a control loop. Despite the control loop, error amplifier 402 is nonetheless susceptible to permitting overshoot by output voltage VOUT2 since a fixed reference voltage is being referenced rather than a reference voltage that gradually ramps upward. In some instances, the dynamics for the control loop for error amplifier circuit 402 can be redesigned, tweaked or otherwise reconfigured to address the overshoot conditions, but such attempts tend to compromise the overall amplifier circuit, and/or result in additional complexity. For example, a P-channel input device for error amplifier 402 can be implemented, with degrading of the overall noise performance of amplifier circuit 400 as a result.